QSC-MX1500-pwr-sm 维修电路原理图.pdf

RadioFans.CN 收音机爱 好者资料库TABLE OF CONTENTS MX1500 Specifications . 3 MX1500 Printed Circlrit Layotit & Parts List . 4 . h4X2500 Component Plac6merit 5 . MX 1 500 Ckaiinel Module Assembly Details -6 . MX 1500 Schematic 7 . MX1 500 Primary Wiring Information . , . 8 . MX1500 Chassis Disassembly 9 . MX2000 Specifications 10 . MX2000 Compotent Placement 11 MX2000 Schematic . 12 . MX2000 Priiary Wiring Information 13 . MX 1500 AMPLIFIER CIRCUIT DESCRIPTION 14 . MXI 500 & MX2000 CIRCUIT DESCRIPTION 14 . Power Supply 14 . Complementary Output Tralsistors 15 . . Complemerttary Driver Transistors, Crossover Bias, and HighILors iver circuit 15 . . Balanced Illput Circuit. Gain Coltrol, and Frequency Limits . . 16 . Amplifier Feedback Network, Gain Stage, Distortion LED, atld S11o: :lircuit Protection 17 . Output Protection and Signal Readout Circuits 18 . Input/Oltput Coniections 1 9 . TROUBLESHOOT1 NG THE MX 1 500/MX2000 A MPLIFTER 20 . Excessive Current Draw -20 . Signal Amplification Problems 20 . Poor Freqieicy Response 21 . Gain Wrong 22 . Incorrect Short Circrrit Current Litnits 22 . D.C. Fault Without Current Draw 22 . Protection Circuit Troubleshooting 22 . Relay Wont Come On 23 . . No Red Protect LED . , 23 . No Mlting Delay 23 . Excess Muting Delay 23 . No Tliennal Sliutoff 23 . No DC Protect 23 . . Relay Falts With Nornally Operating Amplifier . 24 RadioFans.CN 收音机爱 好者资料库MX15QQ Specifications OUTPUT POWER (per channel): Continuous Average Output Power both channels driven. 8 ohms, 20 - 2OkHz 0.1% THD 330 8 ohms, I kHz 1% THD360 4 ohms, 20 - 20khz 0.1% THD 500 4 ohms, I kHz 1% THD570 2 ohms, I kHz 1% THD 750+/-IdB BRIDGED MONO OPERATION: 16ohms,20-2OkHz 0.1% THD660 16 ohms, I kHz 1% THD720 8 ohms, 20 - 2OkHz 0.1% THD 1000 8 ohms, I kHz 1% THD1100 4 ohms, I kHz 1% THD 1500+/-IdB DISTORTION (8 ohms): THD, 20-20kHz, from 250milliwatts to rated power, less that 0.1%, 0.015% typical. SMPTE-IMD, less than 0.02%, 250 milliwatts to rated power. FREQUENCY RESPONSE: 20-ZOkHz, +/-0.1 dB. 8-300kHz, +0/-3 dB. DAMPING FACTOR: Greater than 200. SLEW RATE: 20V per microsecond. DYNAMIC HEADROOM: 3dB or 1000 watts instantaneous power 4 ohms. NOISE: -1 OOdB, 20-20kHZ. SENSITIVITY: 1V RMS for rated power (8 ohms). INPUT IMPEDANCE: 10k unbalanced , 20k balanced. DIMENSIONS: 3 5 tall (2 r a k spaces), 19 wide, 17 9 deep WEIGHT: 471bs net, 52 Ibs shipping MX1500 Component Placement NIX1 500 Channel Module Assembly Dr ails MX1500 Schematic MX1500 Primary Wiring Information MX2000 Specifications OUTPUT POWER (per channel): Continuous Average Output Power both channels driven. 8 ohms, 20 - 20kHz 0.1% THD 375 8 ohms, ?kHz 1% THD425 4 ohms, 20 - 20kHz 0.1% THD 600 4 ohms, 1 kHz 1% THD725 2 ohms, I kHz 1% THD 1000 +I- IdB BRIDGED MONO OPERATION: 16 ohms, 20 - 2OkHz 0.1 % THD 750 8 ohms, 20 - 20kHz 0.1% THD 1200 4 ohms, I kHz 1 % THD 2000 +I- IdB DISTORTION (8 ohms): THD, 20-20kHz, at rated power, less that 0.1%. SMPTE-IMD, less than 0.025% at rated power. FREQUENCY RESPONSE: 20-20kHz, +I-0.1 dB. 8-300kHz, +0/-3 dB. DAMPING FACTOR: Greater than 200. SLEW RATE: 20V per microsecond. DYNAMIC HEADROOM: 2dB at 4 ohms. NOISE: -1 OOdB, 20-20kHZ at rated power. SENSITIVITY: 1.07V RMS for rated power (8 ohms). INPUT IMPEDANCE: 10k unbalanced , 20k balanced. DIMENSIONS: 5.25 tall (2 rack spaces), 19 wide, 17.9 deep. WEIGHT: 701bs net, 76 Ibs shipping. 1 I I 4 111 I MX2000 Schematic MX1500 AMPLlFf ER CIRCUIT DESCRIPTION The following circuit description refers to MX1500 and MX2000 products manufactured between 1985 and 1991. However, some reference designators and portions of the circuit descriptions may differ. In principle the MX2000 will follow the MX1500. The reader will want to cross relative part numbers from the MX2000 to the MXI 500. The MX1500 and MX2000 are electrically equivalent to the Series Ill Amplifiers 3500 and 3800. The MXl500 and MX2000 are Class G dual-monaural amplifiers. The power supplies are dual level, center-tapped, balanced, and bipolar. The channel power supplies are completely independent, except for the power cord and switch. The channels have separate supply transformers, circuit breakers, capacitors, and inputloutput wiring. Each channel can operate independently of the other. The 2-step power supply allows these amplifiers to be twice as efficient as conventional designs. The driver and output transistors are subjected to lower voltage and current than traditional designs. This lowers the channel heatsink requirements and reduces component stress. The lower supply voltage is operating at maximum output when the amplifier is producing approximately 1/3 rated power output (worst cr-e thermal condition in conventional designs). With normal music signals, the high vi .nge output components supply current on signal transients. Since these two amplifiers are DC coupled, they havr output relay protection circuits. The protection circuit will prevent DC from reachins the load (speaker) and remove the load when the amplifier is overheating. The ou:ut relay disconnects the amplifier from the load and shorts the load to circuit ground. Please refer to the MX2000 schematic for the component identification numbers in this troubleshooting guide. The MX2000 has similar operation with different component identification numbers. Power Supply The AC power cord is connected to the supply transformers through a thermal (slow-blow) circuit breaker and the AC switch. A row of terminals on a small circuit board allows the transformer primary to be wired for 120V, 220V, or 240V, 50-60Hz operation (the MX1500 requires a special export transformer). There is a separate supply transformer for each channel. The transformer secondary windings are connected to the channel module through an 8-pin connector. The high voltage windings are rectified by B4, and filtered by E8,9,10 (positive) and E l 1 ,12,13 (negative). Half-voltage windings are rectified by B3 and filtered by El 4 (positive) and El 5 (negative). In addition to the usual full-voltage positive and negative rails, we have secondary rails at half the voltage. These are used in the high-efficiency circuit discussed later on. The output transistors collectors are grounded and the audio output is taken from the midpoint of the power supply capacitors. This requires separate filters, rectifiers, and transformer secondaries for each channel, but permits one channel to fail without effecting the other. The +/-I 5V power supply, for the op amp, is derived by using dropping resistors (R33 and R34) and Zener stabilizers (21 3 and 214) from the main supply rails. Complementary Output Transistors The emitters of the positive and negative output transistors are connected to their respective power supply rails through load-sharing emitter resistors. The parallel bases of each bank are bypassed with 22-ohm resistors (R15-18). These pull-up resistors assure positive shut-off of the output transistors. The 500ma currents from the collectors of the driver transistors are connected to the output transistor bases and amplified to about 25A peak. The collectors of all output transistors meet at a common ground. This means we can simply screw the cases of all four banks to a grounded heat sink, with no need for the usual insulating mica. This saves money and improves reliability through better cooling. The speaker output is taken from the midpoint of the power supply capacitors. This means that the audio output voltage is superimposed on top of the DC supply voltages, which must be kept in mind when checking these voltages. Complem en taw Driver Transistors, Crossover Bias, and HighiLo w driver circuit. We use two stages of discrete devices to amplify the output of the op amp to full rated power. With a little added complexity, we have doubled the efficiency of the normal Class-B output stage. Complementary drivers Q1 and Q2 are connected to the op amp through bias diodes D5 and D6. The forward voltage drop of the diodes matches the foward base voltage of Q1 and Q2. As the op amp swings above and below zero, it immediately drives Q l or Q2. This minimizes the dead zone which causes crossover distortion. Trimmer TI in parallel with R14 permits the bias to be fine-tuned. Emitter resistors R25 and R26 are used to stabilize the gain of Q1 and Q2. These resistors minimize the driver transistors tendency to draw more idle current as they heat up. The final collector current from Q1 and Q2 is about 500 ma. This current is drawn from the bases of the lower voltage output transistors Q5-7 and (211-13. The low voltage output transistors handle the first 50% of the output voltage. In order to deliver the remaining 50% of the output swing, we need to activate the high- voltage drivers Q3 and Q4. These transistors drive the high voltage output transistors (28-10 and Q14-16. This is done by putting Q3 and Q4 in series with Q1 and Q2, and using the 3.9V zeners Z1 5 and Z16 for turn-on threshold. To explain this, we will look at just the positive half of the circuit. Keep in mind that the same things will happen on the negative half-cycle. As the positive driver Q1 is driven harder, its collector current will pull the positive half-voltage power supply closer to ground. Since the entire supply is coupled together as a unit, this means all voltages move negative together. The speaker is connected to !he common midpoint, so this results in a negative speaker output. As we approach the point of 50% output, the positive half voltage supply will come within a few volts of ground. This carries the emitter of QO within a few volts of ground, until its base voltage drops below the voltage established by the zener (215). Q23, Dl I , & Dl2 become forward biased and begin to drive Q3. Q3 takes over the current coming from Q1, cutting off the low-voltage outputs and driving the high-voltage outputs. The output devices pull the high-voltage supply to ground, resulting in 100% output to the speakers. Since the half-voltage supply is carried befow ground during this period, high-current diode D l 1 protects the low-voltage outputs from reverse polarity. This reduces waste heat considerably, compared to the normal single-stage circuit. A power amp with a single level DC supply has a lot of voltage across the power transistors for all levels up to full output. This means that more power is wasted at moderate power levels than at full output. With lower level DC supplies for lower levels of output voltages (up to 50%), we reduce waste voltage and power. The 215 zener voltage (about 3.9V) is added to the overall saturation loss and reduces peak output voltage by this amount. The small transistor after the zener (Q23) isolates the loading effect of Q3 and Q4. This permits full signal swing and improves dynamic headroom. It is difficult to switch smoothly between high and low power stages. The RC network R23 and C7 makes Q3 switch sooner at very high frequencies, improving the step performance. Coil L1, between the low and high-voltage filter capacitors, slightly cushions the transition. Balanced lnput Circuit, Gain Control, and Frequency Limits. The first stage of the dual op amp is used as a balanced differential input. Any signal appearing equally on both the positive and negative inputs results in no voltage across the op amp input terminals and no voltage at the output of the op amp. Noise signals, which normally occur equally on both sides of the balanced line, are rejected. The audio signal appears as a difference between the balanced-line conductors and these signals appear at the same gain at the output of the op amp. For unbalanced inputs, either input line may be grci.inded, and the circuit will respond at unity gain to the other line. For reasons of overall stability in actual use, QSC uses the inverting input for unbalanced signals. After balanced-to-unbalanced conversion, the audio signal flows through R5 and C1, which roll off high frequency response (less than -.2dB at ZOKHz, and -3dB at 200KHz). Amplifier Feedback Network, Gain Stage, Distortion LED, and Short Circuit Protection. The actual power amplifier begins at the input to the second stage of the op amp. Audio feedback is established across R7 and R8. A 47uF non-polar capacitor (E4) in series with R8, ensures that 100% of the DC feedback Goes to the op amp. These components also roll off low frequency response below 20Hz and ensure only a few millivolts of DC offset at the amplifier output. The feedback dements R9 and C4 reduce the feedback below 50Hz. This counteracts the rolloff of E4 in the region of 20- 30Hz. This circuit maintains flat audio response without unnecessary sub-audio bandwidth. The resultant low frequency response is down 0.2dB at 20tlz and 3dB at 8 Hz. High frequency stability is established by C3 and C2. C3 is the primary phase lag capacitor, which sets the slew rate of the circuit. C2 establishes phase lead in the speaker feedback network, increases feedback at very high frequencies, and improves control at frequencies where oscillations might occur. C9 and C10 add a little more phase-lead margin. A speaker output filter, L2, R49, R50, and C5 isolates the amplifier from reactive loads at very high frequencies where instability might occur. Most of the gain in the amplifier is contributed by the second stage of the op amp. This stages primary job is to feed the driver transistors, but is also used to drive the Distortion LED (LD-1) and is tied into the short circuit protection scheme. As long as the output of the amp is not clipping, the drive voltage to the bases of the driver transistors (Q1 and Q2) remains below 1.5 volts peak. At clipping the op amp will attempt to deliver a higher than normal voltage to the drivep transistors. This excess voltage is used to drive the Distortion LED (LD-2) through smE:i bridge rectifier B1. The THD circuit requires a voltage greater than 2.6 volts to operate. Any form of clipping immediately results in illumination of the Distortion LED. In order to maintain good audio performance into low impedance loads, it is necessary to maintain a high output current limit. The normal current limit is determined by the base current to each driver through resistors and Zener diodes (R19 R22, 21 5, and Z16). When the op amp supply rails are at their normal 15V, this currr 7t is about 9ma and results in about a 25A current from the output transistors. -he output transistors can withstand this much current into a shorted output for a few sp -:ends. The normal range of currents from the op amp is less than the current st,plied from the main power supply rails by R33 and R34. On full power operation more current is drawn than these resistors can supply, especially if the amp is clipped and the op amp has to deliver extra current to the Distortion LED. To prevent the op amp rail voltage from being drawn down, we have a replenishing circuit from the speaker output through R52 (three parallel resistors in early versions) and rectifiers D8